I agree with @Drewski, defederation should be the last choice. Kbin does have an option where you can block whole domains, so if you'd prefer that, go here and click the block button: kbin.social/d/fanaticus.social
No, kbin.cafe is not run by the same person that runs kbin.social; here's a post by the owner of kbin.cafe and @ernest is the owner of kbin.social and the main developer of kbin. As you mentioned, kbin works similar to an email service, so different kbins (and lemmys and mastodons) can communicate with each other.